What is a dichotomous key?

• A biological device (tool) which enables one to identify an organism by progressively opting between two alternative observable characteristics

Necessity of using a dichotomous key

• used to identify organisms quickly and accurately
• by following the statements in the key we are able to identify each organism on the basis of a characteristic which is not to be found in other specimens

Rules followed in constructing a dichotomous key

• use observable characteristics only
• start with major characteristics, placing organisms into two groups at each stage
• use a single characteristics at a time
• use contrasting characteristics at each stage e.g 1(a) short, 1(b) tall
• avoid repeating the same characteristics

Procedure of using a dichotomous key and a list of major features of the characteristics to be identified

• look at the features of similarities
• look at the features of differences between the organisms
• we can then be able to identify the organisms by distinguishing one from another
• the key uses a method of elimination by following statements that are correct only for the organism

Example
i)You are provided with a specimen leaves(Araicaria, Cynodon, Grevellea, Kalanchoa, Brassica, Tradescantia, Commelina). Use the dichotomous key below to identify the taxonomic group to which the specimen belongs. Show the steps (number and letter) in the key that you followed to arrive at the identify of the specimen

1 a) leaf broad ...................................................................................................................................go to 2
b) leaf narrow ..................................................................................................................................Araicaria

2 a) leaf parallel veined .................................................................................................................Cynodon
b) leaf net-veined ............................................................................................................................go to 3

3 a) leaf with one lobe (simple) ...................................................................................................go to 4
b) leaf with many lobes (compound) .....................................................................................Grevellea

4 a) leaf fleshy ........................................................................................................................................Kalanchoa
b) leaf not fleshy ...............................................................................................................................go to 5

5 a) leaf petiole modified to form sheath................................................................................ go to 6
b) leaf petiole not modified to form sheath ......................................................................Brassica

6 a) leaf purple .....................................................................................................................................Tradescantia
b) leaf green ....................................................................................................................................Commelina

Steps – 1a, 2b, 3a, 4b, 5b Identify – Brassica

ii) You have been provided with four animals labeled K (mature adult housefly), L (mature adult grasshopper, M(maize flour beetle) and N(worker termite) use the dichotomous key below to identify the specimens. Write down in the correct order, the steps (number and letter) in the key that you followed to arrive at your answer.

Dichotomous key

1 a) animal with wings.............................................................................................................................go to 2
b) animal without wings.................................................................................................................... go to 7

2 a) with two pairs of wings ............................................................................................................... go to 3
b) with one pair of wings ................................................................................................................. Diptera

3 a) with membranous wings ............................................................................................................go to 4
b) hind pair of membranous wings ...........................................................................................go to 6

4 a) with long abdomen ..................................................................................................................Odontata
b) medium sized abdomen ............................................................................................................go to 5

5 a) wings with coloured scales............................................................................................. Lepidoptera
b) wings without scales...................................................................................................... hymenoptera

6 a) forewings hard and shell-like ...................................................................................... coleopteran
b) forewings hard but not shell-like................................................................................ Orthoptera

7 a) body horizontally flattened ................................................................................................... Isoptera
b) body laterally flattened...........................................................................................Symphonoptera

Identify the orders o the various specimens as per the table below

Specimen	order	steps followed	identity
K	Diptera	1a,2b	Housefly
L	Orthoptera	1a, 2a, 3b, 6b	Grasshopper
M	Coleoptera	1a, 2a, 3b, 6a	Beetle
N	Isoptera	1b, 7a	Termite